Telecommunications networks currently rely to a large extent upon Signalling System No. 7 (SS7) as the mechanism for controlling call connections and for handling the transfer of signalling information between signalling points of the networks. Typically, one or more application and user parts at a given signalling point will make use of SS7 to communicate with peer application and user parts at some other signalling point. Examples of user parts are ISUP (ISDN User Part) and TUP (Telephony User Part) whilst examples of application parts are INAP (Intelligent Network Application Part) and MAP (Mobile Application Part). The conventional SS7 protocol stack includes Message Transfer Parts MTP1, MTP2, and MTPee3 which handle the formatting of signalling messages for transport over the physical layer as well as various routing functions. Both signalling and user data is carried over Synchronous Transfer Mechanism (STM) networks using either the E.1 (Europe) or T.1 (USA) systems. In some cases a common STM network is used for both signalling and user data whilst in other cases separate STM networks are used.
There has been considerable interest of late amongst the telecommunications community in using non-standard (i.e. non-conventional within the telecommunications industry) signalling and user data transport mechanisms in telecommunications networks in place of the conventional mechanisms. The reasons for this are related both to improvements in efficiency as well as potential cost savings. Much consideration has been given for example to the use of Internet Protocol (IP) networks to transport signalling and user data between network nodes. IP networks have the advantage that they make efficient use of transmission resources by using packet switching and are relatively low in cost due to the widespread use of the technology (as opposed to specialised telecommunication technology). There is also interest in using other transport mechanisms including ATM (AAL1/2/5), FR etc.
ISUP, which deals with the setting-up and control of call connections in a telecommunications network, is closely linked to the E.1/T.1 STM transport mechanisms and does not readily lend itself to use with non-standard transport technologies such as IP and ATM. As such, several standardisation bodies including the ITU-T, ETSI, and ANSI, are currently considering the specification of a signalling protocol for the control of calls, which is independent of the underlying transport mechanism. This is illustrated in FIG. 1 and can be viewed as separating out from the signalling protocol, Bearer Control functions which relate merely to establishing the parameters (including the start and end points) of the “pipe” via which user plane data is transported between nodes, and which are specific to the transport mechanism. The new protocol, referred to as Bearer Independent Call Control (BICC) or Transport Independent Call Control (TICC), retains Call Control functions such as the services invoked for a call between given calling and called parties (e.g. call forwarding), and the overall routing of user plane data. It is noted that signalling traffic at the Call Control level may be sent over a network (IP, ATM, SS7, etc) which is separate from the network over which Bearer Control signalling traffic and user data is sent. However, in some cases a single shared network may be used. As well as TICC, alternative transport independent call control protocols exist including SIP.
The new network architecture resulting from the separation of the Call and Bearer Control levels results in an open interface appearing between a Call Control entity and a Bearer Control entity, where these entities are referred to as a Media Gateway Controller and a Media Gateway respectively. The open interface is referred to hereinafter as X-CP, examples of which are the MEGACO work of the IETF and the H.248 work of ITU Study Group 16 (SG16). It is envisaged that a given Media Gateway Controller may control several Media Gateways (or indeed a Media Gateway may be controlled by several Media Gateway Controllers).
FIG. 2 illustrates schematically a situation where a telecommunications network operator uses a first ATM network to carry user data and Bearer Control signalling traffic, and a second ATM network to carry Call Control signalling traffic (in FIG. 2 not all protocol layers and other functionalities are shown for the sake of clarity). It is necessary for the operator to provide an interface to a conventional telecommunications network (e.g. a Public Switched Telephone Network (PSTN)), and this is achieved via a Media Gateway Controller 1, a Signalling Gateway 2, and a Media Gateway 3.
The BICC entity of the MGC 1 sits on top of an MTP1 to 3b entity which in turn sits on top of an AAL2 entity. These layers allow communications with peer MGCs over the ATM network. The MGC 1 communicates with the MG 3 using a Gateway Control Protocol (GCP) over an IP interface (or using another suitable protocol). In addition, the MGC 2 has ISUP, TCAP, and SCCP entities which enable communication with PSTN signalling points. Call Control related signalling traffic is transported between the MGC 2 and the PSTN through the SG 2, with the SG terminating the MTP layers 1 to 3. The MG 3 comprises, in addition to the GCP and AAL2 entities, an MTP layer 1 to 3b entity which enables the setting up of connections over the ATM bearer network (e.g. using B-ISUP). The MG 3 is also connected to the PSTN to send user data to, and receive user data from, the PSTN. In the MG 3, the block identified as “MG functionality” indicates the intelligent functions performed by the MG 3.